1. Field of the Invention
The present invention relates to an electronic device having a structure for ohmic connection to a carbon element cylindrical structure body represented by a carbon nanotube and a production method thereof, which techniques are widely applicable when a carbon element cylindrical structure body is applied to an electronic device.
The present invention also relates to a method for growing carbon nanotubes.
2. Description of the Related Art
In recent years, studies are being made on the use of a carbon element cylindrical structure body as an electrically conducting or semiconductor material in an electronic device. In the electronic device, a carbon element cylindrical structure body must be ohmically connected to an electrode or a wire so as to avoid an increase in electric resistance in the connection part.
FIG. 1 schematically shows the connection of a carbon element cylindrical structure body 11, which is formed as a longitudinal via material in a via hole of an electronic device, to a lower wiring Cu layer 13 and an upper wiring Cu layer 15. In a via hole formed in an interlayer insulating film 17 on the lower wiring layer 13, a bundle of carbon element cylindrical structure bodies 11 is formed in vertical orientation. The carbon element cylindrical structure body 11 is grown by using a chemical vapor deposition (CVD) method and, at this time, a catalyst metal (for example, Ni) layer 19, necessary for the growth of the carbon element cylindrical structure body, is present on the wiring layer exposed in the via hole. On the other hand, between the carbon element cylindrical structure body 11 and the upper wiring layer 15, a Ti layer 21 is inserted.
The connection between the carbon element cylindrical structure body 11 and the upper wiring layer 15 is an ohmic connection resulting from carbidization (formation into TiC) of the Ti layer 21 intervening therebetween. This method is a technique of contacting the carbon element cylindrical structure body with the Ti layer and then performing a heat treatment at a high temperature to cause TiC formation at the interface, thereby obtaining an ohmic connection (see, Y. Zhang et al., Science 285, 1719 (1999)).
As shown in FIG. 1, in the conventional structure, only a catalyst metal (Ni) layer 19, necessary for the growth of the carbon element cylindrical structure body 11, is present on the lower wiring (Cu) layer 13.
With respect to the production method of a carbon nanotube, arc discharge, laser evaporation, thermal CVD, plasma enhanced CVD and the like are known. By the arc discharge or laser evaporation method, a carbon nanotube having good quality can be obtained but the orientation or length of carbon nanotubes can hardly be controlled.
The method of enabling the control of the orientation or direction includes a thermal CVD method and a plasma enhanced CVD method. In these methods, carbon nanotubes can be orientation-grown by applying an electric field during the growth. The orientation growth in the case of not applying an electric field is described in Nature, Vol. 416, pp. 495-496 (2002), however, the growth temperature is as high as 800° C. or more and therefore, it is impossible to grow carbon nanotubes on a semiconductor circuit by this technique. Furthermore, the growth of carbon nanotubes at 550° C. is reported in Chemical Physics Letters, 360, pp. 2229-234 (2002), however, the growth direction cannot be controlled.
Various methods for producing carbon nanotubes by using thermal CVD are described in patent documents. For example, Japanese Unexamined Patent Publication (Kokai) No. 9-31757 (JP 9-31757 A) discloses a method of producing graphite nanotubes at a low temperature by CVD, where the graphite nanotube is produced at 650 to 800° C. Japanese Unexamined Patent Publication (Kokai) No. 10-203810 (JP 10-203810 A) describes a technique of growing carbon nanotubes on a substrate at a relatively low temperature, where the growth requires a plasma produced by a direct-current glow discharge. Japanese Unexamined Patent Publication (Kokai) No. 11-139815 (JP 11-139815 A) describes a method for producing a carbon nanotube device by using a thermal decomposition reaction of the starting material gas. Also, Japanese Unexamined Patent Publication (Kokai) No. 2001-303250 (JP 2001-303250 A) describes a method of vertically orienting carbon nanotubes on a substrate by using thermal CVD, where a direct current voltage is applied during the growth.
In these methods for producing carbon nanotubes by using thermal CVD, when auxiliary means such as an electric field is not used, a growth temperature of 500° C. or more is generally used.
A CVD method using a hot filament (hot-filament CVD) is also known. Japanese Unexamined Patent Publication (Kokai) No. 2000-353467 (JP 2000-353467 A) describes a method for producing a cold cathode device, where a diamond or diamond-like carbon electron-releasing material is formed by hot-filament CVD. The production of carbon nanotubes is not referred to therein. Japanese National Publication (Kohyo) No. 2002-518280 (JP 2002-518280 A) describes a method for growing carbon nanotubes by hot-filament CVD. In this method, an electric field is applied during the growth.
In this way, in conventional production of carbon nanotubes by hot-filament CVD, application of an electric field is performed as auxiliary means.
Also, a technique of growing carbon nanotubes at a filament temperature of 1,600° C. by hot-filament CVD is described in Chemical Physics Letters, 342, pp. 259-264 (2001).